Hazardous waste site cleanup is a major environmental concern on which significant manpower and resources are expended. Many such hazardous waste sites exist which are an immediate environmental concern. Typically, these hazardous waste sites were created by the dumping of hazardous chemicals in inadequately designed dump pits or sites. The chemicals were deposited in bulk or in leaking or non-corrosion resistant containers. The chemicals at these sites were thus free to seep into the underlying soils and into underlying aquifers. The movement of the contaminants within the soil and aquifers has resulted in enlarging the contaminated area beyond the actual dump site.
One method of decontaminating the hazardous waste sites was to completely remove the contaminated soil by excavation. The removed material was then either treated at a processing facility or transported to another land fill site from which the spread of contaminants was more easily controlled. This method was very expensive and time consuming. Moreover, transporting the contaminated soil from one site to another only postponed the eventual treatment.
Another method for mitigating ground water contamination was the isolation of the hazardous waste site. Clay or synthetic caps were placed over the source of contamination and the plume of contaminants to minimize the infiltration of water from precipitation into the soil. Although the caps had limited downward migration, they had not significantly affected the rate or direction of natural groundwater flow. Caps had also been used in conjunction with slurry wells. In this method, wells were drilled on the perimeter of the contaminant plume and slurry was injected into the subsurface soil. The slurry typically contained bentonite or other materials to reduce soil permeability. However, slurry wells did not completely prevent horizontal movement of contaminants and long term reliability had not been proven. Moreover isolation only delayed detoxification and the waste site remained a potential environmental hazard.
Another attempt at mitigating groundwater contamination has been fluid removal. In fluid removal systems, both drains and wells have been used. Typically, the use of drains involved excavating a pit located toward the downstream end of the contaminant plume. The pit was dug to a depth below the contaminated level and a skimmer was used to collect contaminants from the surface of the water accumulating in the pit. The drain system has been limited in application to shallow plumes in low permeability soils. Since drains were generally exposed to the surface, their application has not been desirable in flood-prone areas. Moreover, removal of contaminants with drain systems has been slow, often requiring many years to reduce the contaminants to an environmentally acceptable concentration.
As an alternative to drains, the drilling of contaminant removal wells provided some flexibility with respect to depth and removal rate. However, the installation of wells was costly and, as with drains located toward the downstream perimeter of the contaminant plume, complete removal of the contaminants could take many years. Accelerated remedial action in which a number of wells were drilled into the source and plume of contaminants hastened the removal of contaminants, but the cost of such systems was extremely high.
More recently, it has been proposed to treat the contaminants in situ using biological or chemical treatment to immobilize or detoxify the contaminants. With this proposal it has been difficult to achieve uniform distribution and mixing of the biological or chemical agents with the contaminants in the subsurface soil. For this proposed method to be effective, wells for injecting the agents must be spaced very closely, and the method has been generally impracticable because of the high cost involved. Thus, all of the methods of the prior art are either ineffective to treat the total contamination problem, or are not practical due to reliability, costs or treatment time involved.